exit: move wake_up_all() pidfd waiters into __unhash_process()

Move the pidfd notification out of __change_pid() and into
__unhash_process(). The only valid call to __change_pid() with a NULL
argument and PIDTYPE_PID is from __unhash_process(). This is a lot more
obvious than calling it from __change_pid().

Link: https://lore.kernel.org/[email protected]
Reviewed-by: Oleg Nesterov <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

kernel/exit.c

@@ -133,8 +133,13 @@ struct release_task_post {
 static void __unhash_process(struct release_task_post *post, struct task_struct *p,
 			     bool group_dead)
 {
+	struct pid *pid = task_pid(p);
+
 	nr_threads--;
+
 	detach_pid(post->pids, p, PIDTYPE_PID);
+	wake_up_all(&pid->wait_pidfd);
+
 	if (group_dead) {
 		detach_pid(post->pids, p, PIDTYPE_TGID);
 		detach_pid(post->pids, p, PIDTYPE_PGID);

kernel/pid.c

@@ -359,11 +359,6 @@ static void __change_pid(struct pid **pids, struct task_struct *task,
 	hlist_del_rcu(&task->pid_links[type]);
 	*pid_ptr = new;
 
-	if (type == PIDTYPE_PID) {
-		WARN_ON_ONCE(pid_has_task(pid, PIDTYPE_PID));
-		wake_up_all(&pid->wait_pidfd);
-	}
-
 	for (tmp = PIDTYPE_MAX; --tmp >= 0; )
 		if (pid_has_task(pid, tmp))
 			return;